I am not responsible for investigating the source of bitcoins.  They are fungible.  There are mixers.  That makes it impossible to do.  The fiat cash in your pocket has been used for multiple drug trades, prostitution, etc.  Do you have a moral obligation to burn it?  Would it change anything?

I don't make change for strangers at all unless it is business. It is estimated that fully a third of all cash is counterfeit. I prefer to handle as little as possible.

No?

Remorse for what? Voluntary trade of value? I would however avoid trade with people I believe aren't working towards the same goal as me, the goal of living in a society that is free to maximize it's potential. In fact I'd actively ostracize such people.

You've got it all mixed up... Really sketchy guys you should be worried about wear suits and offer you lines of credit, bright future, and fight against "terrorists" from your OP. Do you ask them about the origin of their money? You should. You should gather facts before attempting to come up with  the "moral obligations" you brought up in OP.

But with fiat currencies one can morally justify accepting most cash (such as supermarket money etc...) without much thought because one can assume (possibly falsely) that there are some kind of checks and balances behind it.

Whether it's true not you can feel like it's not your problem because there are government agencies, companies and banks and you can dismiss the responsibility by thinking "I'm sure they're checking it all out".

But when El Binhamidi Boom Boom comes to you with $20,000 worth of bitcoins for 75% market price, you know there is no one but you between him and cash.  And without you he may not have been able to get cash because an international wire transfer may have been flagged, or cash in his underwear on the plane may have been found, etc...

I'm mostly focusing on terrorism because I think most people would find it morally disturbing so makes for clearer examples.

Because with fiat money laundering schemes it's fairly easy to see whether you are a part of a scheme or not (yeah if you are accepting payments from random people and forwarding 80% you know you are laundering money).  But with Bitcoin one can fall into a grey area where you are not really sure what's going on with the money around you, and you can keep it that way if you want.

The guy could have been busy selling tamagotchis for bitcoins, and he's just cashing out.

Or he could be building a dungeon in his basement for 2 year olds.

He could easily do both. Financing his dungeon with his tamagotchis sells.

But I get what you mean. So, lets replace that example in a way that it actually makes sense:

The guy could have been busy selling tamagotchis for bitcoins, and he's just cashing out.

Or he could have been busy selling slaves and he's just cashing out.

And still I see no difference in doing this with BTC than with any other currency. If I'm exchanging $ to Yen or $ to BTC, there are the same obligations involved.

But when trading $ to Yen you would probably be required by law in your country to register as a currency trader and you would be required to do a certain amount of reporting and due diligence.  Such as requiring valid ID for exchange, reporting amounts over a certain amount etc... etc...so it kind of answers the question of obligations for you, as the government tells you exactly what your obligations are and then you can just kind of forget about it and brush it off as "I did what I was supposed to do"

This is very true. You HAVE moral obligations when accepting bitcoins.

In fact, in order to clear your conscience, you have to send the 10% here 1JuZoErGwTucetm7Ac33aA9wfjkn2eASza